I need some advice. I could get 2 gtx 580 OC 1.5gb for around $450 . Would this still be a good investment? or just get either a single 670 or 660Ti? or even a 680? it's very tempting because it's like half of what it used to cost (per card) but I understand that they've already discontinued this model.
Also, how much of a power beast are these cards? i understand under load they are around 500 - 600 Watts but translated to elect bill terms; would the increased power consumption cost be very considerable?

The 580 SLI setup is probably faster than the 680 at times, mainly when SLI scales well and the 1.5vram isn't bottlenecking you. It's going to suck about twice the power too although what that means to your power bill doesn't mean much unless you fold or game 24/7.

If yes then 1 580 is enough (or 660).. the raw power of sli 580's will beat any single core card available at the moment..
but as said above, how that translates to game-play with power usage, heat, noise and how sell the game scales with SLI would point me to a single card solution.

I wouldn't upgrade a 580 at the moment if I had one.. next gen and then I would consider it (at 1080)
If you looking to buy a whole new GPU solution, then I would go single 670 or for high res stuff 7970..

Go 580's if you want to play with SLI and the challenges it will bring..

Remember you need pretty good cooling and a reasonable PSU for dual 580's (850 would be my suggestion) so take that into account too.

At that res, probably not. I haven't read much lately about vram usage but 6 months ago the majority felt that 1.5-2gb was not bottlenecking anything other than BF3 and a few other games at very high res/settings. I would probably not think twice about a 2gb card myself... but if you have thoughts of going to 1440p or 120hz monitors the 4gb might be nice insurance.

If for example i'm going for triple monitors, but still use 60 hz monitors, would going for higher vram really help? for such a set up would a gtx 670 4gb be the same, performance wise as a gtx 680 2gb?
Pardon my inexperience moving up to the Big boy sandbox is confusing..

Hard to say... I kinda think trying to run triple monitors with a single card is stretching unless you don't mind moderate settings. I guess the question is what games are you going to play, and how good do you want it to look?

I'm into FPS and more so into driving games. Will definitely go initially into a single card setup and 1 monitor then move on to triple and drop in another card to SLI. 2gb cards are cheaper; just dont want to kick myself by taking that and find out that i needed more vram. however, dont want to spend more if the extra vram is not really needed..